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International System of Units overhauled in historic vote

npl.co.uk

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Re: International System of Units overhauled in historic vote

#171

Post by jd007 »

keketi wrote: post #11
xaduha wrote: post #6

> a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It will be replaced by the Planck constant – the fundamental constant of quantum physics. I thought they did that long time ago.

I wonder how much the cylinder will auction for.

Last I heard, the original Big K, along with (some of) the secondary copies, will not be sold. Instead, they will be observed/studied in an on-going basis, in order to better understand why their masses appear to have drifted apart over the years (which is partially the reason for this re-definition).

Re: International System of Units overhauled in historic vote

#173

Post by maccam94 »

jackpirate wrote: post #153
jcranmer wrote: post #114

Earlier quoted context omitted.

Understanding mass in molar terms is necessary to do a lot of chemistry correctly. The mole is effectively a count of the number of molecules kicking about, although the count is large enough that terms like "quadrillion" don't cut it. (One mole is about 600 sextillion molecules). Knowing how many molecules are around lets you actually compute how much stuff can react in a given chemical environment, and other aspect…

I think the GP is making the claim that Avogadro's constant should be 1 (effectively eliminating it), not that there's no need to count molecules.

An Avogadro's constant number of molecules is one mole. One mole of hydrogen has much less mass than one mole of iron. You have to choose an arbitrary mass of a single element as the base quantity. Hydrogen might be the best theoretically, it's just a proton and an electron, but it is tricky to work with because it's a gas at room temperature. So instead the mole has been defined as the number of molecules in a specific mass of carbon-12.

Re: International System of Units overhauled in historic vote

#174

Post by garmaine »

3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

Well, now you could do that. At least you could next March when these rules take affect. But before you could not because the conversion between atomic mass and SI/kg mass depended on that experimental constant. Two mass systems were required because we couldn’t conver between them with atomic accuracy.

Re: International System of Units overhauled in historic vote

#175

Post by kevin_thibedeau »

clord wrote: post #135
rubenhak wrote: post #117

Finally there is order in units for science community. Hopefully such transformation will come to US one day.

The US units are defined in terms of SI since the Mendenhall Order[0], so they also benefit from this change. [0]: https://en.wikipedia.org/wiki/Mendenhall_Order

It wasn't until 1959 that the yard was metricized. This still affects precision machinery manufactured before the changeover.

Re: International System of Units overhauled in historic vote

#176

Post by garmaine »

Zarel wrote: post #22
3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

It's not a fundamental constant. > the Avogadro number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of atomic hydrogen, meaning one gram of hydrogen. (from Wikipedia) (It's since been refined to be 12 grams of carbon-12.) So a mole is defined to be approximately one gram worth of protons and neutrons. We use it because grams are a significantly easier unit of mass for huma…

Read the OP. It is now a fundamental constant. (But you’re right if you’re referring to the status quo before).

Re: International System of Units overhauled in historic vote

#177

Post by kevin_thibedeau »

anonytrary wrote: post #157
xvilka wrote: post #5

Too bad US doesn't use them.

Anyone who is doing real work in the US uses SI units. The only case where you're using "feet", "miles", "pounds", etc. is in everyday language.

NASA and NOAA scientists and engineers still use customary units.

Re: International System of Units overhauled in historic vote

#178

Post by [deleted] »

pfbtgom wrote: post #83

How would you use the new definition to calibrate instruments? My current understanding is that there is a lineage of artifacts calibrated against another artifact until one of those was calibrated against the one true artifact. So how would NIST or another certifying source say that my 1kg standard is 1kg +/- tolerance? Is it anyone with a kibble balance can now certify calibrations? How do you know your kibble bala…

[deleted]

Re: International System of Units overhauled in historic vote

#179

Post by coder543 »

simcop2387 wrote: post #115
pfbtgom wrote: post #83

How would you use the new definition to calibrate instruments? My current understanding is that there is a lineage of artifacts calibrated against another artifact until one of those was calibrated against the one true artifact. So how would NIST or another certifying source say that my 1kg standard is 1kg +/- tolerance? Is it anyone with a kibble balance can now certify calibrations? How do you know your kibble bala…

Essentially this brings it one more step away from using another artifact for calibrating things. Instead this lets us define the kilogram against what we believe are universal constants, in this case properties about the electromagnetic force. It's much simpler to understand looking at it with a watt balance, even though it's not going to be as precise or accurate as a kibble balance [1]. Basically now anyone with a…

I'm pretty sure a watt balance and a Kibble balance are the same thing. Regardless, that YouTube video is so good! I was just about to post the same link.

Re: International System of Units overhauled in historic vote

#180

Post by mjevans »

simcop2387 wrote: post #115
pfbtgom wrote: post #83

How would you use the new definition to calibrate instruments? My current understanding is that there is a lineage of artifacts calibrated against another artifact until one of those was calibrated against the one true artifact. So how would NIST or another certifying source say that my 1kg standard is 1kg +/- tolerance? Is it anyone with a kibble balance can now certify calibrations? How do you know your kibble bala…

Essentially this brings it one more step away from using another artifact for calibrating things. Instead this lets us define the kilogram against what we believe are universal constants, in this case properties about the electromagnetic force. It's much simpler to understand looking at it with a watt balance, even though it's not going to be as precise or accurate as a kibble balance [1]. Basically now anyone with a…

Or potentially more usefully, ascertain an accurate measurement of how closely a given measuring device is to the target to calibrate it's use in actual measurements (outside of the expensive validation mechanisms).
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